CH 102 Lecture Notes - Lecture 3: Lattice Energy, Wood Frog, Intermolecular Force

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Solutions:
Unlike most cold-blooded animals, wood frogs have the ability to survive very cold temperatures
by hibernating in a suspended "frozen" state
o In its partially frozen state, a wood frog has no heartbeat, no blood circulation, no breath,
and no brain activity
o Their vital functions can be revived within 1-2 hours of thawing
Most cold-blooded animals cannot survive freezing temperatures because the water within their
cells freezes and frozen water (ice) expands, destroying or causing irreversible damage to cells
Wood frogs survive freezing temperatures because of their physiological ability to "flood their
blood" with a large amount of glucose (sugar water solution) prior to going into hibernation
o The high concentration of glucose in the bloodstream prevents the blood from freezing
because concentrated solutions have a lower freezing point than the corresponding pure
liquid
Solutions are homogeneous mixtures
o Two or more substances make up a mixture
o A solution may be composed of a solid and a liquid, a gas and a liquid, or other combinations
Solutions have a solute component and a solvent component
o The majority component of a solution is called the solvent
o The minority component is called the solute
The solute is dissolved into the solvent
Solution formation is the result of the interaction of the intermolecular forces of solute and
solvent particles
o Likes dissolve in like
Nature has a tendency toward spontaneous mixing
o Generally, uniform mixing is more energetically favorable
When one substance (solute) dissolves in another (solvent), it is said to be soluble
o Salt is soluble in water
o Bromine is soluble in methylene chloride
When one substance does not dissolve in another, it is said to be insoluble
o Oil is insoluble in water
The solubility of one substance in another depends on the following:
o Nature's tendency toward mixing
o The types of intermolecular attractive forces
When solutions with different solute concentrations come in contact, they spontaneously mix to
result in a uniform distribution of solute throughout the solution
Many physical systems tend toward lower potential energy
o Formation of a solution does not necessarily lower the potential energy of the system
When two ideal gases are put into the same container, they spontaneously mix, even though the
difference in attractive forces is negligible
o The gases mix because the energy of the system is lowered through the release of entropy
Entropy is the measure of energy dispersal throughout the system (disorder)
Energy has a spontaneous drive to spread out over as large a volume as it is allowed
o As each gas expands to fill the container, it spreads its energy out and lowers its entropy
Energy changes in the formation of most solutions also involve differences in attractive forces
between the particles
o Dispersion
o Dipole-dipole
o Hydrogen bond
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o Ion-dipole
For the solvent and solute to mix, you must overcome:
o All of the solute-solute attractive forces; or
o Some of the solvent-solvent attractive forces
Both processes are endothermic
At least some of the energy to do this comes from making new solute-solvent attractions, which is
an exothermic process
o When the solute-to-solvent attractions are weaker than the sum of the solute-to-solute and
solvent-to-solvent attractions, the solution will form only if the energy difference is small
enough to be overcome by the increase in entropy from mixing
The maximum amount of solute that can be dissolved in a given amount of solvent is called the
solubility
o There is usually a limit to the solubility of one substance in another
Gases are always soluble in each other
Two liquids that are mutually soluble are said to be miscible
Alcohol and water are miscible
Oil and water are immiscible
The solubility of one substance in another varies with temperature and pressure
A chemical will dissolve in a solvent if it has a structure similar to that of the solvent
o Compatible intermolecular forces
Polar molecules and ionic compounds will be more soluble in polar solvents
Nonpolar molecules will be more soluble in nonpolar solvents
When some compounds, such as NaOH, dissolve in water, a lot of heat is released
o The container gets hot
When other compounds, such as NH4NO3, dissolve in water, heat is absorbed from the
surroundings
o The container gets cold
To make a solution, you need to do the following:
o Oeoe all attatios etee the solute patiles; theefoe, ΔHsolute is endothermic
o Oeoe soe attatios etee solet oleules; theefoe, ΔHsolvent is endothermic
o Form new attractions between solute particles and solvent oleules; theefoe, ΔHmix is
exothermic
The oeall ΔH fo akig a solutio depeds o the elatie sizes of the ΔH fo these thee
processes
o ΔHsoln = ΔHsolute + ΔHsolvent + ΔHmix
If the total energy cost for breaking attractions between particles in the pure solute and pure
solvent is less than the energy released in making the new attractions between the solute and
solvent, the overall process will be exothermic
If the total energy cost for breaking attractions between particles in the pure solute and pure
solvent is greater than the energy released in making the new attractions between the solute and
solvent, the overall process will be endothermic
For aqueous solutions of ionic compounds, the energy added to overcome the attractions
between water molecules and the energy released in forming attractions between the water
molecules and ions are combined into a term called the heat of hydration
o Attractive forces between ions = lattice energy
ΔHsolute = -ΔHlattice energy
o Attractive forces in water = H bonds
o Attractive forces between ion and water = ion-dipole
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